"had better, should, or ought to." Also can be used in front of any popular phrase, or as an adverb or subjunctive to describe an action or noun.
That's fidda crazy!
Fidda get rollin'!
Fidda check yourself, before you wreck yourself.
Note that when using this word it may also feel correct and necessary to spell out the last word of the sentence in which fidda is used, and if it is a two-word sentence then you may only spell out that second and last word without saying it.
(Fidda funk that junk, j-u-n-k. OR Fidda r-o-l-l.